This invention relates to a cationic electro-coating composition, or, in more detail, to a cationic electro-coating composition which contains no lead compounds and is nevertheless capable of forming an electrocoated film which is excellent in corrosion resistance, appearance, etc.
Electropaint not only has good throwing power, but also is capable of forming a coated film which is excellent in durability and corrosion resistance. Hence, electropaint has widely been employed in fields which require these properties, e.g., in the coating of automobile body, the coating of electrical appliances, etc.
Anticorrosion agent such as lead compound and chrome compound have been blended in electropaint so as to further improve its corrosion resistance. These anticorrosion agents are very harmful, and, in view of measures against public pollution problems, it has been undesirable to use them. On this account, various studies have been made to seek non-toxic or low toxic anticorrosion agents in place of the above-mentioned lead compound and chrome compound. Now bismuth compound is known as one which exhibits excellent corrosion resistance.
For instance, Japanese Patent Application Laid-Open (Kokai) No. Hei 5 (1993)-65439 (corresponding to U.S. Pat. Nos. 5,330,839 and EP-A-509 437) proposes a cationically electrodepositable coating composition which contains bismuth compound. The use of such an electropaint gives rise to the formation of a coated film which is excellent in corrosion resistance and low temperature curability. However, when bismuth compound is not grinded uniformly enough, there occur problems that corrosion resistance and low temperature curability are insufficient, or that precipitates are apt to be produced in paint.
Japanese Patent Application National Publication No. Hei 7 (1995)-506870 (corresponding to WO 93/24578), on the other hand, discloses an electrodepositable catalyst-containing cation paint binder which contains a bismuth salt of aliphatic hydroxycarboxylic acid. This binder contains a large amount of aliphatic hydroxycarboxylic acid to retain the water-solubility of bismuth salt. Hence, an electropaint which is prepared with use of this binder has acid in excess, resulting in problems that electrocoating workability such as throwing power and film appearance markedly decreases.
In order to solve the above-mentioned problem, Japanese Patent Application Laid-Open (Kokai) No. Hei 8 (1996)-60046 (corresponding to U.S. Pat. No. 5,670,441 and EP-A-690 106) proposes to use, in combination with epoxy-amine adduct, a bismuth compound which is prepared with use of acid in such an amount as gives less than two moles of dissociative proton per mole of bismuth. Japanese Patent Application National Publication No. Hei 9 (1997)-505837 (corresponding to WO 95/07377), on the other hand, proposes to disperse, in a paint binder, a mixture of bismuthyl lactate with bismuth lactate which mixture is prepared from a reaction between bismuth oxide and lactic acid by a special method. In the former proposal, however, bismuth compound is apt to precipitate in paint, while, in the latter, the amount of acid used cannot be reduced to a full extent, resulting in a problem that thus obtained electropaint is insufficient in electrocoating workability such as throwing power and film appearance.
The inventors of this invention assiduously studied with a view to overcoming the above-mentioned problems, and, as a result, have found out that either the use of an aqueous dispersion which contains non water-soluble organic acid-modified bismuth compound which is obtained by mixing and grinding a bismuth compound together with an organic acid in an aqueous medium or the use of an aqueous dispersion paste which contains non water-soluble organic acid-modified bismuth compound which is obtained by mixing and grinding, in the presence of a dispersant, a bismuth compound together with an organic acid in an aqueous medium makes it possible to uniformly and stably disperse, in electropaint, non water-soluble organic acid-modified bismuth compound, and thus makes it possible to form, without using a lead compound etc., an electro-coated film which is excellent in both appearance and corrosion resistance. The inventors of this invention have thus completed the present invention.
This invention provides a cationic electro-coating composition which comprises blending an aqueous dispersion which contains an organic acid-modified bismuth compound in a non water-soluble state, and which is obtained by mixing and grinding a bismuth compound together with an organic acid in an aqueous medium.
This invention also provides a cationic electro-coating composition which comprises blending an aqueous dispersion paste which contains an organic acid-modified bismuth compound in a non water-soluble state, and which is obtained by mixing and grinding, in the presence of a dispersant, a bismuth compound together with an organic acid in an aqueous medium.
The cationic electro-coating composition of this invention is explained in more detail in the following.
Bismuth Aqueous Dispersion and Bismuth Aqueous Dispersion Paste
The aqueous dispersion (hereinafter referred to as bismuth aqueous dispersion), to be blended in the cationic electro-coating composition according to this invention, which contains non water-soluble organic acid-modified bismuth compound is produced by mixing and grinding a bismuth compound together with an organic acid in an aqueous medium. Said organic acid is used in such a proportion that non water-soluble organic acid-modified bismuth compound may be formed as a main product. Said mixing and grinding of a bismuth compound and an organic acid in an aqueous medium are desirably conducted with use of a grinding mill at a temperature of about 50xc2x0 C. or lower. In this manner, there is obtained an aqueous dispersion of bismuth in which thus formed organic acid-modified bismuth compound is dispersed stably in a non water-soluble, cloudy and creamy state. When blended with an electropaint, this aqueous dispersion of bismuth remarkably improves curability and corrosion resistance without spoiling throwing power and appearance of electrocoated film.
The aqueous dispersion paste (hereinafter referred to as bismuth aqueous dispersion paste or simply aqueous dispersion paste), to be blended in the cationic electro-coating composition according to this invention, which contains non water-soluble organic acid-modified bismuth compound is produced by mixing and grinding, in the presence of a dispersant, a bismuth compound together with an organic acid in an aqueous medium. Said organic acid is used in such a proportion that non water-soluble organic acid-modified bismuth compound may be formed as a main product. If water-soluble organic acid-modified bismuth compound is produced in a large amount, bismuth becomes hard-of depositing into a coated film when electrocoated. Even though the amount of water-soluble organic acid-modified bismuth compound added is increased, sufficient corrosion resistance is hard to be obtained especially when electrocoated film has a small thickness. In this manner, there is obtained an aqueous dispersion paste of bismuth in which thus formed organic acid-modified bismuth compound is dispersed stably in a non water-soluble and cloudy state. When blended with an electropaint, this aqueous dispersion paste of bismuth remarkably improves curability and corrosion resistance without spoiling throwing power and appearance of electrocoated film.
The above-mentioned bismuth aqueous dispersion and bismuth aqueous dispersion paste may contain water-soluble bismuth compound. It is desirable, however, that the content of said water-soluble bismuth compound is such that the content of water-soluble bismuth compound which exists in supernatant obtained by centrifuging said bismuth aqueous dispersion or bismuth aqueous dispersion paste at 12000 rpm for 30 minutes may be, as metal bismuth by weight, at most about 40%, in particular at most about 30%, more desirably at most about 20%, based on the total amount of non water-soluble bismuth compound which is used as a raw material.
Examples of bismuth compound which is used for the preparation of such a bismuth aqueous dispersion or bismuth aqueous dispersion paste include basic bismuth compound such as bismuth oxide, bismuth hydroxide and basic bismuth carbonate, in which bismuth oxide is in particular suitable.
Organic acid is used for the purpose of changing the above-mentioned bismuth compound into a sufficiently uniform and stable dispersion in non water-soluble state in an aqueous medium. Examples of said organic acid include organic carboxylic acid such as formic acid, acetic acid, lactic acid and propionic acid; and organic sulfonic acid such as amidosulfonic acid. Especially preferred is aliphatic carboxylic acid which has the following formula: 
wherein R1 denotes a hydrogen atom or an alkyl group having 1 to 3 carbon atoms, R2 denotes a hydrogen atom or an alkyl group having 1 to 10 carbon atoms, and n denotes 0 or 1.
Concrete examples of said aliphatic carboxylic acid include aliphatic hydroxycarboxylic acid such as hydroxyacetic acid, lactic acid and hydroxypropionic acid; and aliphatic alkoxycarboxylic acid such as methoxyacetic acid, ethoxyacetic acid, 3-methoxypropionic acid, among which especially preferable are lactic acid, in particular L-lactic acid, and methoxyacetic acid. These acids may be used either singly or in combination of two or more species. Said aliphatic carboxylic acid may be used together with other organic acid mentioned above.
The above-mentioned organic acid is used in such an amount that the obtained organic acid-modified bismuth compound may be able to exist in a non water-soluble state in an aqueous medium. Although said amount varies according to the species of organic acid used, said organic acid is generally used in a molar ratio of 0.25-2.5, preferably 0.5-1.7, more desirably 0.75-1.3, based on the amount of bismuth in bismuth compound. When L-lactic acid is employed as an organic acid, it may usually be used in a molar ratio of 0.5-0.7, preferably 0.75-1.3, based on the amount of bismuth in bismuth compound. When methoxyacetic acid is employed as an organic acid, it may usually be used in a molar ratio of 0.25-2.5, preferably 0.75-1.3, based on the amount of bismuth in bismuth compound.
As a dispersant which is used for the preparation of bismuth aqueous dispersion paste, there is used a known cationic dispersion resin or surface active agent without any restriction. Said cationic dispersion resin may be appropriately selected from among those which are mentioned later as base resin for electropaint. Examples of such a cationic dispersion resin include tertiary amine type, quaternary ammonium salt type and tertiary sulfonium salt type resin. Examples of surface active agent include acetylene glycol type, polyethylene glycol type or polyhydric alcohol type nonionic surfactant whose HLB is in the range of 3-18, preferably 5-15.
The amount of the above-mentioned dispersant used may be varied according to its species or the amount of bismuth compound used or the like. Usually, however, the dispersant is used in an amount of 1-150 parts by weight, especially 10-100 parts by weight, in particular 50-100 parts by weight, based on 100 parts by weight of bismuth compound.
The preparation of bismuth aqueous dispersion with use of the above-mentioned bismuth compound and organic acid may be carried out, for instance, by adding organic acid and bismuth compound to water, and subjecting the resultant mixture to a grinding treatment in a grinding machine such as ball mill or sand mill at a temperature of about 50xc2x0 C. or lower. Thus obtained aqueous dispersion may usually have a solid content of 1-50% by weight, preferably 5-30% by weight.
The preparation of bismuth aqueous dispersion paste with use of bismuth compound, organic acid and dispersant may be conducted in the same manner as in the preparation of pigment paste which is used for cationic electro paint. Concretely, it may be carried out, for instance, by adding organic acid and bismuth compound to water which contains dispersant, and subjecting the resultant mixture to a grinding treatment in a grinding machine such as ball mill or sand mill. Thus obtained aqueous dispersion paste may usually have a solid content of 10-70% by weight, preferably 30-60% by weight.
Bismuth aqueous dispersion or bismuth aqueous dispersion paste may be prepared as a pigment paste by adding pigments which are used for usual cationic electro-coating. Concretely, pigment dispersion resin, neutralizing agent and pigments, for instance, are blended, and the resultant mixture is subjected to a grinding treatment in a grinding machine such as ball mill or sand mill to prepare a pigment paste, to which the above-mentioned bismuth aqueous dispersion or bismuth aqueous dispersion paste may be added. Examples of said neutralizing agent to neutralize the above-mentioned pigment dispersion resin include organic acid such as acetic acid and formic acid.
As the above-mentioned pigment dispersion resin, any known one may be employed without restriction. Concretely, the same cationic dispersion resin as is used for the preparation of the above-mentioned bismuth dispersion paste may be employed.
As the above-mentioned pigments, any known one may be used without restriction so long as it is a pigment which is usually used for electropaint. Concrete examples include coloring pigments such as titanium oxide, carbon black and blood red; extender pigments such as clay, mica, baryta, talc, calcium carbonate and silica; and anticorrosion pigments such as aluminum phosphomolybdate and aluminum tripolyphosphate.
Bismuth aqueous dispersion or bismuth aqueous dispersion paste or a pigment paste which contains either of them may be blended in bider resin component etc. of cationic electro-coating. Bismuth aqueous dispersion may also be added after dispersing an electropaint in water.
The above-mentioned bismuth aqueous dispersion or bismuth aqueous dispersion paste may generally be blended in an electro-coating composition in such a proportion that bismuth content may fall within a range of 0.1 to 10 parts by weight, preferably 0.3 to 7 parts by weight, more desirably 0.5 to 5 parts by weight, based on 100 parts by weight of resin solid content of electropaint.
Electro-coating Composition
An electro-coating composition in which the above-mentioned bismuth aqueous dispersion or bismuth aqueous dispersion paste is blended basically comprises a cationizable base resin and, if necessary, a curing agent. Suitable examples of said base resin include epoxy-, acrylic- and polyurethane-resins which contain cationizable functional group, among which polyamine resin which is represented by amine-added epoxy resin is especially preferred in view of corrosion resistance.
Examples of the above-mentioned amine-added epoxy resin include (i) an adduct of epoxy resin with primary mono- and polyamine, secondary mono- and polyamine or with a mixture of primary and secondary polyamines (see: U.S. Pat. No. 3,984,299); (ii) an adduct of epoxy resin with secondary mono- and polyamine having ketiminized primary amino group (see: U.S. Pat. No. 4,017,438); (iii) an etherification reaction product between epoxide resin and a hydroxy compound having ketiminized primary amino group [see: Japanese Patent Application Laid-Open No. Sho 59 (1984)-430131].
The epoxy resin which is used for the production of the above-mentioned amine-added epoxy resin is a compound which has at least two epoxy groups per molecule, and which suitably has a number average molecular weight of at least 200, preferably 400 to 4000, more desirably 800 to 2000. In particular suitable are a compound obtained from a reaction between polyphenol compound and epichlorohydrin, and a compound obtained by making an adduct of polyphenol compound with alkylene oxide react with epichlorohydrin. Examples of polyphenol compound which is used for the formation of said epoxy resin include bis(4-hydroxyphenyl)-2,2-propane; 4,4-di-hydroxybenzophenone; bis(4-hydroxyphenyl)-1,1-ethane; bis(4-hydroxyphenyl)-1,1-isobutane; bis(4-hydroxy-tert-butyl-phenyl)-2,2-propane; bis(2-hydroxynaphtyl)methane; tetra(4-hydroxyphenyl)-1,1,2,2-ethane; 4,4-dihydroxydiphenylsulfone; phenolic novolak; and cresol novolak.
Said epoxy resin may be a product of partial reaction with polyol, polyetherpolyol, polyesterpolyol, polyamide amine, polycarboxylic acid or polyisocyanate compound, or may be a product of graft polymerization of xcex5-caprolactone, acryl monomer, etc. Furthermore, a reaction product from a reaction between polyol and epichlorohydrin, a product of reaction between said reaction product and polyphenol compound, or a product which is obtained by oxidizing an unsaturated compound with peracetic acid may be used as the above-mentioned epoxy resin.
The above-mentioned base resin may be either an external-crosslinking type or an internal (or self)-crosslinking type. As a curing agent which is used for an external-crosslinking type resin, any known crosslinking agent may be employed. Although blocked polyisocyanate compound is especially preferred, tris(alkoxycarbonylamino)triazine is also usable. An internal-crosslinking type resin has preferably blocked isocyanate groups introduced therein.
Blocked polyisocyanate compound which is usable with the above-mentioned external-crosslinking type base resin may be a product of an addition reaction between polyisocyanate compound and isocyanate blocking agent. Examples of said polyisocyanate compound include aromatic, alicyclic or aliphatic diisocyanate compound such as tolylene diisocyanate, xylylene diisocyanate, phenylene diisocyanate, bis(isocyanatomethyl)cyclohexane, tetramethylene diisocyanate, hexamethylene diisocyanate, methylene diisocyanate and isophorone diisocyanate, and isocyanurate thereof, and further include termimally isocyanate-containing compound which is obtained by making an excess amount of said isocyanate compound react with a low molecular active hydrogen-containing compound such as ethylene glycol, propylene glycol, trimethylol propane or hexane triol.
The above-mentioned isocyanate blocking agent add to, and temporarily block, isocyanate group of polyisocyanate compound. It is desirable that blocked polyisocyanate compound which is thus formed as a result of said addition is stable at a normal temperature, and capable of reproduce a free isocyanate group by dissociating the blocking agent when heated to a baking temperature of about 100 to 200xc2x0 C. Examples of blocking agent which satisfies such requirements include lactam compound such as xcex5-caprolactam and xcex3-butyrolactam, etc.; oxime compound such as methylethyl ketoxime and cyclobexanone oxime, etc.; phenolic compound such as phenol, para-t-butyl-phenol and cresol, etc.; aliphatic alcohol such as n-butanol and -2-ethylhexanol, etc.; aromatic alkylalcohols such as phenylcarbinol and methylphenylcarbinol, etc.; and etheralcohols such as ethylene glycol monobutyl ether, among which oxime type and lactam type blocking agents, which dissociate at a considerably low temperature, are in particular preferable in view of curability of electrodeposition coating composition.
The introduction of blocked isocyanate group into a self-crosslinking type base resin which has blocked isocyanate group in the molecule of base resin may be conducted by any known method. For instance, said introduction is achieved by making a free isocyanate group in partially blocked polyisocyanate compound react with active hydrogen-containing portion of base resin.
Base resin can be rendered water-soluble or water-dispersible by neutralizing the above-mentioned base resin with a water-soluble organic acid such as aliphatic carboxylic acid, in particular acetic acid or formic acid. Acetic acid and formic acid are preferably used as a neutralizing agent since they give an electropaint which is excellent in appearance, throwing power and low temperature curability.
The electro-coating composition of the present invention may, if necessary, contain tin compound as a curing catalyst. Examples of said tin compound include organotin oxide such as dibutyl tin oxide and dioctyl tin oxide; and aliphatic or aromatic carboxylic acid salt of dialkyltin such as dibutyl tin laurate, dioctyl tin dilaurate, dibutyl tin diacetate, dioctyl tin benzoate oxy, dibutyl tin benzoate oxy, dioctyl tin dibenzoate and dibutyl tin dibenzoate. The content of tin compound in the electro-coating composition is not strictly specified, and may vary widely according to performance which is required of electropaint. Usually, however, tin compound is preferably contained so that tin content may fall within the range of 0 to 8 parts by weight, desirably 0.05 to 5 parts by weight, more desirably 0.5 to 3 parts by weight, per 100 parts by weight of resin solid content in electropaint.
The electro-coating composition of the present invention may, if necessary, contain zinc compound as an anticorrosion agent. Examples of said zinc compound include zinc phosphate, zinc formate, zinc acetate, zinc molybdate, zinc oxide and zinc phosphomolybdate. The content of zinc compound in the electro-coating composition is not strictly specified, and may vary widely according to performance which is required of electropaint. Usually, however, zinc compound is preferably contained so that zinc content may fall within the range of 0 to 8 parts by weight, desirably 0.05 to 5 parts by weight, more desirably 0.1 to 3 parts by weight, per 100 parts by weight of resin solid content in electropaint.
Furthermore, the electro-coating composition of the present invention may, if necessary, contain a paint additive such as organic solvent and surface-conditioner.
The electro-coating composition of the present invention can be applied onto the surface of a desired metal substrate by means of electrodeposition coating. Said electrodeposition coating can be conducted as follows: The electro-coating composition of the present invention is diluted with deionized water or the like so that solid content may be about 5 to 40% by weight, and is adjusted so that pH may fall within the range of 5.0 to 9.0. Thus formed electro-coating bath is normally set at a bath temperature of 15 to 35xc2x0 C., and a voltage of 100 to 400 V is applied to the electro-coating bath.
The electrocoated film thickness which can be formed with use of the electrodepositable coating composition of the present invention is not restricted in particular. Preferred thickness is generally within a range of 10 to 40 xcexcm on the basis of cured coated film. Preferable baking temperature of coated film is generally in the range of 100 to 200xc2x0 C.